Transformers are used in many types of electronic device to perform such functions as transforming impedances, linking single-ended circuitry with balanced circuitry or vice versa and providing electrical isolation. However, not all transformers have all of these properties. For example, an autotransformer does not provide electrical isolation.
Transformers operating at audio and radio frequencies up to VHF are commonly built as coupled primary and secondary windings around a high permeability core. Current in the windings generates a magnetic flux. The core contains the magnetic flux and increases the coupling between the windings. A transformer operable in this frequency range can also be realized using an optical-coupler. An opto-coupler used in this mode is referred to in the art as an opto-isolator.
In transformers based on coupled windings or opto-couplers, the input electrical signal is converted to a different form (i.e., a magnetic flux or photons) that interacts with an appropriate transforming structure (i.e., another winding or a light detector), and is re-constituted as an electrical signal at the output. For example, an opto-coupler converts an input electrical signal to photons using a light-emitting diode. The photons pass through an optical fiber or free space that provides isolation. A photodiode illuminated by the photons generates an output electrical signal from the photon stream. The output electrical signal is a replica of the input electrical signal.
At UHF and microwave frequencies, coil-based transformers become impractical due to such factors as losses in the core, losses in the windings, capacitance between the windings, and a difficulty to make them small enough to prevent wavelength-related problems. Transformers for such frequencies are based on quarter-wavelength transmission lines, e.g., Marchand type, series input/parallel output connected lines, etc. Transformers also exist that are based on micro-machined coupled coils sets and are small enough that wavelength effects are unimportant. However such transformers have issues with high insertion loss and low primary to secondary isolation and low primary to secondary isolation.
All the transformers just described for use at UHF and microwave frequencies have dimensions that make them less desirable for use in modern miniature, high-density applications such as cellular telephones. Such transformers also tend to be high in cost because they are not capable of being manufactured by a batch process and because they are essentially an off-chip solution. Moreover, although such transformers typically have a bandwidth that is acceptable for use in cellular telephones, they typically have an insertion loss greater than 1 dB, which is too high.
Opto-couplers are not used at UHF and microwave frequencies due to the junction capacitance of the input LED, non-linearities inherent in the photodetector, limited power handling capability and insufficient isolation to give good common mode rejection.
Above-mentioned U.S. patent application Ser. No. 10/699,481, of which this disclosure is a continuation-in-part, discloses a film acoustically-coupled transformer (FACT) based on decoupled stacked bulk acoustic resonators (DSBARs). A DSBAR is composed of a stacked pair of film bulk acoustic resonators (FBARs) and an acoustic decoupler between the FBARs. FIG. 1A schematically illustrates an embodiment 100 of such FACT. FACT 100 has a first DSBAR 106 and a second DSBAR 108 suspended above a cavity 104 in a substrate 102. DSBAR 106 has a lower FBAR 110, an upper FBAR 120 stacked on lower FBAR 110, and an acoustic coupler 130 between the FBARs, and DSBAR 108 has a lower FBAR 150, an upper FBAR 160 stacked on lower FBAR 150, and an acoustic coupler 170 between the FBARs. Each of the FBARs has opposed planar electrodes and a piezoelectric element between the electrodes. FBAR 110 has opposed planar electrodes 112 and 114 with a piezoelectric element 116 between them. FBAR 120 has opposed planar electrodes 122 and 124 with a piezoelectric element 126 between them.
FACT 100 additionally has a first electrical circuit 141 interconnecting the lower FBAR 110 of DSBAR 106 and the lower FBAR 150 of DSBAR 108 and a second electrical circuit 142 interconnecting the upper FBAR 120 of DSBAR 106 to the upper FBAR 160 of DSBAR 108.
In the embodiment of the above-described FACT shown in FIG. 1A, FBARs 110, 120, 150 and 160 are all nominally equal in impedance and electrical circuit 141 connects lower FBARs 110 and 150 in anti-parallel and to terminals 143 and 144 and electrical circuit 142 connects upper FBARs 120 and 160 in series between terminals 145 and 146. Electrical circuit 142 additionally has a center-tap terminal 147 connected to electrodes 122 and 162 of upper FBARs 120 and 160, respectively. This embodiment has a 1:4 impedance transformation ratio between electrical circuit 141 and electrical circuit 142 or a 4:1 impedance transformation ratio between electrical circuit 142 and electrical circuit 141.
In other embodiments of FACT 100, FBARs 110, 120, 150 and 160 are all nominally equal in impedance, electrical circuit 141 electrically connects the lower FBARs either in anti-parallel or in series, and electrical circuit 142 electrically connects the upper FBARs either in anti-parallel or in series. The possible combinations of electrical circuit configurations just described are summarized in Table 1 below:
TABLE 1ParallelSeriesAnti-parallel.Anti-seriesParallelU 1:1 LOWXXU 1:4SeriesXB 1:1 HIB 4:1XAnti-parallel.XB 1:4B 1:1 LOWXAnti-seriesU 4:1XXU 1:1 HI
In Table 1, the row captions indicate the configuration of electrical circuit 141, the column captions indicate the configuration of electrical circuit 142, B denotes that the FACT is electrically balanced, U denotes that the FACT is unbalanced, and X denotes a non-functioning FACT. The impedance transformation ratio shown is the impedance transformation from the configuration of electrical circuit 141 indicated by the row caption to the configuration of electrical circuit 142 indicated by the column caption. For the configurations having a 1:1 transformation ratio, LOW denotes that the FACT has a low impedance, equivalent to that of two FBARs in parallel, and HI indicates that the FACT has a high impedance, equivalent to that of two FBARs in series.
Inspection of Table 1 indicates that embodiments of the above-described FACT 100 have impedance transformation ratios of 1:1 low impedance, 1:1 high impedance or 1:4 (low impedance to high impedance), generally 1:2n, where n=1 or 2. In this disclosure, a transformation ratio of 1:m will be understood to encompass a transformation ratio of m:1, since a FACT with a transformation ratio of 1:m can be converted into a FACT with a transformation ratio of m:1 simply by interchanging the input and output terminals.
While FACT embodiments having impedance transformation ratios of 1:1 or 1:4 are useful in many applications, other applications need different impedance transformation ratios. What is needed, therefore, is a FACT that has the advantages of the FACT described above, but that has an impedance transformation ratio different from 1:1 or 1:4.